This invention relates to a photo-curable epoxy resin composition, and more particularly to a photo-curable epoxy resin composition which has improved curing and electric characteristics and which is suitable for use as insulating and resist materials in electric equipment.
Recently, processes for hardening the resins by use of light are attracting attention to save energy and improve operating efficiency in the field of resins. Among others, processes for photo-curing epoxy resins are regarded as important because of the wide range of their possible applications. In the photocurable processes for epoxy resins, the compositions of epoxy resins themselves are very important, besides the curing conditions. Thus various compounding compositions of epoxy resins have broadly been studied. To date, two types of photo-curable epoxy resin compositions have been known; one is an epoxy resin which is modified by the addition of a photo-polymerizable compound containing the double bond or bonds, such as acrylic acid and its derivatives, and the other is a composition containing the photo-decomposing type catalysts such as those having the formula .phi..sub.3 S.sup..sym. PF.sub.6.sup..crclbar. or .phi..sub.2 X.sup..sym. BF.sub.4.sup..crclbar. in which .phi. represents a phenyl group and X represents a halogen [Japanese Patent Publication Nos. 14277/1977, 14278/1977 and 14279/1977; Japanese Provisional Patent Publication Nos. 95686/1979, 125104/1980 and 125105/1980, and U.S. Pat. No. 4,081,276]. Photo-cured products obtained from the former type, i.e., modified epoxy resins, show heat resistance considerably inferior to that of photo-cured products obtained from epoxy resins themselves. On the other hand, the latter type, i.e., the epoxy resin compositions containing the photo-decomposing type catalysts, give photo-cured products which show good mechanical and thermal characteristics. In this case, however, the catalyst components remain as ionic impurities in the photo-cured products, adversely affecting the electric characteristics thereof. In addition, when such photo-cured products containing the ionic impurities are used in electric equipment, the ionic impurities sometimes cause the electric equipment to corrode.